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What's the Difference Between AT&T and Verizon's 3G Networks?

Verizon made it official today: it will be getting the iPhone, ending years of AT&T exclusivity in the U.S. With the Verizon iPhone a reality, there is one question that millions of iPhone owners are now asking themselves: should I switch carriers?

After years of frustration with dropped calls and spotty 3G connectivity, we understand why so many iPhone users are eager to become Verizon customers. In our reader poll, 29% of Mashable readers said they're switching to Verizon ASAP, while another 16% said they're switching after their current contract expires.

A lot of people have been asking us about whether switching is a good idea. More specifically, they want to understand the differences between the AT&T iPhone, a UMTS/GSM device, and the Verizon iPhone, a CDMA gadget. What exactly is CDMA? Is it better or worse than UMTS? Why is it impossible to to talk and browse the web at the same time on a Verizon iPhone?

To answer your questions, we wrote a short guide on the subject of CDMA, UMTS and the differences between AT&T and Verizon's 3G networks:

The Basics on CDMA, GSM and UMTS

The primary difference between the AT&T iPhone and the Verizon iPhone is the technology each uses for 3G connectivity.

AT&T employs the Universal Mobile Telecommunications System in its network. UMTS is built upon concepts from the Global System for Mobile Communications (GSM) standard, the basis for EDGE (Enhanced Data rates for GSM Evolution). Because of this shared architecture, most UMTS devices support GSM and EDGE, including the AT&T iPhone.

GSM and UMTS technology is widely used worldwide. UMTS phones can be easily moved from one UMTS network to another, making them ideal for international use.

Verizon, on the other hand, uses code division multiples access (CDMA), a form of spread-spectrum signaling that utilizes bandwidth more effectively than most other solutions; it's often considered more spectrally efficient. CDMA also utilizes soft handovers of phone calls, so instead of switching directly from one tower to the next (hard handover), CDMA devices can receive a signal from multiple towers simultaneously. That makes the shift between towers less detectable, and it decreases the rate of dropped calls. On the other hand, UMTS devices aren't as efficient at soft handovers.

CDMA technology does have several disadvantages, though. It can't use a SIM card, making it far more difficult to switch handsets. More important is that UMTS networks tend to upload and download data faster than CDMA. A test last year by PCWorlddemonstrated that AT&T has the fastest 3G network by a healthy margin. There is, of course, great variation in network speeds depending on location, population density and other factors, but in general you're going to get a faster 3G connection on AT&T (UMTS) than you will on Verizon (CDMA).

One of the other big disadvantages of CDMA is its inability to carry voice and data simultaneously. AT&T phones can browse the web while on a phone call. Verizon phones don't have that capability, although the network is working on that feature.

What's the Bottom Line?

In the end, your phone is going to work essentially the same on both networks, and the truth is that the vast majority of users won't notice any difference. Here are some key points to remember, though:

Calls: In general, Verizon will drop less calls. It's unclear what impact the iPhone will have on its network, but we don't expect it to be as bad as AT&T was in its early years. Verizon's network has proven itself to be more robust.

Speed: In general, AT&T has the faster 3G network, and in some cases it's a great deal faster than Verizon.

Simultaneous voice and data: Only AT&T is capable of talking on the phone and surfing the web at the same time, but Verizon is working on a solution, saying the fix might be implemented by the end of this year.

The latter point may become a nonissue soon, though. According to The Wall Street Journal, a solution to carry voice and data simultaneously on CDMA networks is coming later this year:

"The inability to multitask can be traced to a decision years ago by the engineers developing CDMA to split voice and data traffic into separate signals, so they can't run both at once.

Now, they're working to overcome it. A solution that will allow CDMA networks to carry voice and data simultaneously will become commercially available in the first half of [2011], said Brad Shewmake, spokesman for the CDMA Development Group, an industry organization."

This argument between CDMA and UMTS may become a moot point in the next few years, though, as 4G networks overtake 3G technology. WiMax and LTE are the primary 4G standards. Both AT&T and Verizon are utilizing LTE as a standard for their 4G networks.

LTE is a GSM-based technology, so it's no surprise AT&T would embrace it. Verizon's decision to utilize LTE though could make the argument between the two networks moot, though. They are using the same technology, meaning that the effectiveness of their 4G implementations will determine how fast and reliable they are. Because it's GSM-based, it will also support simultaneous voice and data.

For now, it's smart to at least know the advantages and disadvantages of CDMA and UMTS. It will change in the next few years, though.

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